2-fluoronitrobenzene is useful as an intermediate in the preparation of certain known herbicides, such as the 2-substituted aryl-4,5,6,7-tetrahydro-2H-isoindole-1,3-diones disclosed in U.S. Pat. No. 4,001,272, granted on Jan. 4, 1977 to Steven J. Goddard.
2-FLUORONITROBENZENE HAS, HERETOFORE, BEEN PREPARED BY THREE DIFFERENT BASIC SYNTHETIC METHODS. The first method involves the nitration of fluorobenzene using a mixture of sulfuric acid and nitric acid. However, the product obtained from this method is an isomeric mixture of 4-fluoronitrobenzene and 2-fluoronitrobenzene, typically in a 9:1 ratio.
The second method involves the diazotization of 2-nitroaniline and conversion to 2-nitrodiazonium fluoroborate. The thermal decomposition of the dry diazonium fluoroborate gives 2-fluoronitrobenzene in only 10-19% yield. The thermal decomposition of diazonium fluoroborates is dangerous and proceeds in low yield when there is a nitro group in the position ortho to the diazonium group.
The third method is based on the halogen exchange of 2-chloronitrobenzene with an alkali metal fluoride. The halogen exchange reaction can be conducted both with and without solvent. Previous methods, however, require long reaction times and high temperatures, give only moderate yields of 2-fluoronitrobenzene, and/or require the use of rare alkali metal fluorides such as cesium or rubidium fluoride.
U.S. Pat. No. 3,240,824 discloses a process for preparing 2-fluoronitrobenzene in 60.6% yield (based on 50.5% conversion) by heating under pressure, equimolar amounts of 2-chloronitrobenzene and potassium fluoride without solvent at 290.degree. C for 24 hours.
Finger and Kruse [J. Amer. Chem. Soc., 78 6034 (1956)] disclose a process for preparing 2-fluoronitrobenzene in dimethylformamide at 170.degree. C for 163 hours in 40% yield, and in dimethylsulfoxide at 185.degree. C for 4.5 hours in 38% yield.
Fukui et al. [Nippon Kagaku Zasski, 79, 889 (1958)];[Chemical Abstracts, 54, 4430 c (1958)] improves upon the yield reported above in a dimethylsulfoxide system. Fukui maintains the reaction temperature at 185.degree. C for 10 hours and reports a 56% yield.
Fuller [German Offenlegungsschrift No. 2,527,944 (1975)] was the first to disclose the preparation of 2-fluoronitrobenzene by reaction of 2-chloronitrobenzene with potassium fluoride in a tetramethylene sulfone (sulfolane) solvent, although others had reported earlier the use of sulfolane as the solvent for fluorination (with potassium fluoride) of other materials. For example, Fuller [J. Chem. Soc., 6264 (1965)] discloses fluorination of hexachlorobenzene and Matsui et al. [Japanese Kokai, 74/110,637] discloses fluorination of 4-chloronitrobenzene (with a catalytic amount of cesium fluoride) in sulfolane. Fuller's preparation of 2-fluoronitrobenzene is conducted at 240.degree. C for 22 hours and results in a 60.5% yield.
All previous methods for preparing 2-fluoronitrobenzene by halogen exchange have one or more of the disadvantages mentioned above; that is, they require long reaction times and/or high temperatures, give only moderate yields of the desired product, and/or require the use of rare alkali metal fluorides such as cesium or rubidium fluoride.